Two stage entrained gasification system and process

ABSTRACT

The present invention relates to a system and process for gasifying feedstock such as carbonaceous materials. The invention includes partial combustion of dry solids and pyrolysis of carbonaceous material slurry in two separate reactor sections and produce mixture products comprising synthesis gas. The invention employs one or more catalytic or sorbent bed for removing tar from the synthesis gas. The inventive system and process allow a gasification to be carried out under higher slurry feeding rate and lower temperature with the provision to manage the tar being produced, therefore to increase the conversion efficiency of the overall gasification.

BACKGROUND OF THE INVENTION

The present invention relates generally to a gasification system andprocess for gasifying feedstock such as carbonaceous materials. Threebasic types of system and processes have been developed for thegasification of carbonaceous materials. They are: (1) fixed-bedgasification, (2) fluidized-bed gasification, and (3) suspension orentrainment gasification. The present invention relates to the thirdtype of system and process—suspension or entrainment gasification. Moreparticularly, the present invention relates to a two stage entrainedgasification system and process for gasifying carbonaceous materials.

Gasification systems and processes are often applied for convertinggenerally solid feedstock such as carbonaceous material into desirablegaseous products such as synthesis gas. Gasification system and processmust be designed to be simple yet to deliver the maximum conversionefficiency.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to a process for gasifying acarbonaceous material comprising the steps of: a) introducing recycledchar into a reactor lower section and partially combusting the recycledchar therein with a stream comprising an oxygen-containing gas and/orsteam thereby evolving heat and forming mixture products comprisingsynthesis gas and molten slag; b) passing the synthesis gas from thereactor lower section upward into a reactor upper section and pyrolysingtherein with a stream comprising a slurry of particulate carbonaceousmaterial in a liquid carrier thereby forming mixture products comprisingi) a gaseous product stream comprising synthesis gas and residual tar,ii) char, and iii) vapor; c) passing the mixture products from thereactor upper section through a separation device whereby the char areseparated from the mixture products from the reactor upper section andrecycled as feed stock to be introduced to the reactor lower section;and d) introducing the gaseous product stream from the reactor uppersection comprising synthesis gas and residual tar into a reaction zonewhereby the residual tar is removed. The heat evolved in the step (a) isrecovered by converting the slurry of particulate carbonaceous materialand the carrier liquid into the gaseous product stream in step (b). Inone embodiment of the present invention, the gaseous product stream fromthe reactor upper section comprising synthesis gas and residual tar areintroduced into a reaction zone comprising one or more catalytic bedcomprising one or more tar-destruction catalysts. In another embodimentof the present invention, the gaseous product stream from the reactorupper section comprising synthesis gas and residual tar are introducedinto a reaction zone comprising one or more sorbent bed comprising oneor more tar-absorbent sorbent.

Another aspect of the present invention relates to a system forgasifying a carbonaceous material comprising: a) a reactor lower sectionfor partially combusting recycled char with a stream comprising anoxygen-containing gas and/or steam to produce heat and mixture productscomprising synthesis gas and molten slag; b) a reactor upper section forpyrolysing the synthesis gas from the reactor lower section with astream comprising a slurry of particulate carbonaceous material in aliquid carrier to produce mixture products comprising i) a gaseousproduct stream comprising synthesis gas and residual tar, ii) char, andiii) vapor; c) a separating device for separating the char from themixture products from the reactor upper section; and d) a reaction zonefor removing residual tar from the gaseous product from the reactorupper section comprising synthesis gas and residual tar. The heatproduced from reactor lower section is recovered by converting theslurry of particulate carbonaceous material and the carrier liquid inreactor upper section into the gaseous product stream in reactor uppersection. The reactor lower section further comprises one or moredispersion devices for introducing the stream comprisingoxygen-containing gas and steam and the recycled char into the reactorlower section. The reactor upper section further comprises one or morefeeding devices for feeding the slurry of particulate carbonaceousmaterial in the liquid carrier into the reactor upper section. Thereactor upper section may be, but not limited to be, positioned abovethe reactor lower section. In one embodiment of the present invention,the reaction zone for removing residual tar from the gaseous productfrom the reactor upper section comprising synthesis gas and residual tarcomprises one or more catalytic bed comprising one or moretar-destruction catalysts. In another embodiment of the presentinvention, the reaction zone for removing residual tar from the gaseousproduct from the reactor upper section comprising synthesis gas andresidual tar comprises one or more sorbent bed comprising one or moretar-absorbent sorbents.

The temperature of reactor lower section is maintained from 1500° F. to3500° F. The pressure in reactor lower section and reactor upper sectionare from about 14.7 psig to about 2000 psig. The velocity of gases andchar passing through the dispersion devices of the reactor lower sectionis from 20 to 120 feet per second. The residence time of char in thereactor lower section is from 2 to 10 seconds. The velocity of theslurry stream passing through the feeding devices of the reactor uppersection is from 5 to 100 feet per second. The residence time of theslurry of the particulate carbonaceous material in the reactor uppersection is from 5 to 40 seconds.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the embodiments of the presentinvention, reference will now be made to the accompanying drawings,wherein:

FIG. 1 is a schematic representation of a system useful in and apictorial process flow diagram for an embodiment in connection with thepresent invention.

FIG. 2 is a schematic representation of a system useful in and apictorial process flow diagram for an alternative embodiment inconnection with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description of various embodiments of theinvention references the accompanying drawings which illustrate specificembodiments in which the invention can be practiced. The embodiments areintended to describe aspects of the invention in sufficient detail toenable those skilled in the art to practice the invention. Otherembodiments can be utilized and changes can be made without departingfrom the scope of the present invention. The following detaileddescription is, therefore, not to be taken in a limiting sense. Thescope of the present invention is defined only by the appended claims,along with the full scope of equivalents to which such claims areentitled.

Referring to FIG. 1, various embodiments of the present inventionprovide a gasification reactor, indicated generally by reference numeral10, has a reactor lower section 30 and a reactor upper section 40. Thefirst stage of the gasification process takes place in the reactor lowersection 30 and the second stage of the gasification process takes placein the reactor upper section 40. The reactor lower section 30 definesthe first stage reaction zone. The reactor lower section 30 will also bereferred to as the first stage reaction zone. The reactor upper section40 defines the second stage reaction zone. The reactor upper section 40will also be referred to as the second stage reaction zone.

Further refer to FIG. 1, the recycled char, and a stream comprising anoxygen-containing gas or steam at high pressure is entered into thegasification reactor 10 lower section 30 through dispersion device 60and/or 60 a, which located, for example, oppositely of the reactor lowersection 30. More than two dispersion devices can be used, for example,four, arranged 90 degrees apart. The sets of dispersion devices can alsobe on different levels and do not need to be on the same plane. Withinthe reactor lower section 30, also the first stage reaction zone, of thegasification reactor 10, the recycled char, and a stream comprising anoxygen-containing gas and/or steam react in such fashion that rapidmixing and reaction of the reactants occur and that a rotating motion ofthe reactants, passing upwardly such as (but not limited as) a vortexthrough the lower section 30 of the reactor 10. The reaction in thereactor lower section 30 is the first stage of the gasification processby which the recycled char, and a stream comprising an oxygen-containinggas or steam are converted exothermically into mixture productscomprising steam, synthesis gas, intermediate gases, and entrainedby-products such as molten slag, at the reaction condition in thereactor lower section 30 as disclosed later in more detail. The moltenslag is drained from the bottom of the reactor 10 through a tap hole 20,to a slag processing system (not shown) for final disposal.

The steam, intermediate, and synthesis gas then exit from reactor lowersection 30 by flowing upward into an unfired reactor upper section 40where a slurry of particulate carbonaceous solids and liquid carrier areinjected through feeding device 80 and/or 80 a, or additional feedingdevices. The heat produced in the reactor lower section 30 and carriedupward with the gas stream is used for the pyrolysis process which takesplace in the unfired reactor upper section 40 (or and the concentrationof these gases in the synthesis gas second stage reaction zone),including vaporization of the feed water, the carbon-steam reaction andthe water-gas reaction between the CO and H₂O. The carbon-steam reactionforms CO and H₂, thus, increasing the yield of these usable gases. Whilethe fired reactor lower section 30 (or the first stage reaction zone ofthe reactor 10) is primarily a combustion reactor, the reactor uppersection 40 is primarily a quench reactor which also increases theheating value of the gases. The reactions occurring in unfired reactorupper section 40 thus enrich the gases emanating from the fired partialcombustion reactor lower section 30 to produce a higher grade ofsynthesis gas and in doing so recover heat from the reactor lowersection 30 and cool the gases sufficiently that the slag entrained iscooled below the ash fusion initial deformation temperature and volatileorganic and inorganic species condense and absorbed on the particulatecarbonaceous material. By cooling to below the ash initial deformationtemperature, the entrained slag droplets fuse by themselves or on theparticulate carbonaceous material prior to reaching the heat transfersurface and therefore do not adhere to heat transfer surfaces. Thereaction condition in the reactor upper section 40 will be disclosed inmore detail later.

In an embodiment of the present invention, as shown in FIG. 1, theunfired reactor upper section 40 of the reactor 10 is connected directlyto the top of the fired reactor lower section 30 of the reactor 10 sothat the hot reaction products are conveyed directly from the reactorlower section 30 to the reactor upper section 40 to minimize heat lossesin the gaseous reaction products and entrained solids.

As illustrated in FIG. 1, the char produced by gasification reaction maybe removed and recycled to increase carbon conversion. For example, charmay be recycled through dispersion devices 60 and/or 60 a (or others)into the reactor lower section, or the first reaction zone as discussedabove.

The dispersion devices 60 and 60 a provide an atomized feed of theparticulate solids such as char. The dispersion devices may be of thetype having a central tube for the solids and an annular spacesurrounding the central tube containing the atomizing gas which opens toa common mixing zone internally or externally. Further, the feedingdevice 80 and/or 80 a of the unfired reactor upper section 40 may alsobe similar to the dispersion devices described hereinabove, or simplyhaving a tube for slurry feeding. Dispersion devices 60, 60 a, orfeeding devices 80, 80 a can be as are conventionally known to thoseskilled in the art.

As further shown in FIG. 1, the mixture products of the second stagereaction occurred in the reactor upper section 40 is withdrawn from thetop of the upper section 40 of reactor 10 and introduced into aseparating device 50 which splits the mixture products into a solidsstream and gas stream, leaving only a small fraction of residual solidfines in the gas stream.

The gas stream comprises hydrogen, carbon monoxide, a small amount ofmethane, hydrogen sulfide, ammonia, water vapor or steam, vapor from theliquid carrier, nitrogen, carbon dioxide and residual tar. The solidsstream comprises solidified ash and char formed in the unfired reactorupper section reactor 40 or carried over from the fired reactor lowersection 30.

The solids stream such as dry char exiting from separating device 50 ismixed with oxygen-containing gas and/or steam and recycled back to theunfired reactor lower section 30 through dispersion devices 60 and/or 60a as feed stock for first stage reaction.

The recycled char is then gasified under slagging conditions by reactionwith oxygen and steam, producing mixture products including synthesisgas and heat required for the second stage reaction within the upperreactor section 40.

The gas stream comprising synthesis gas, residual char fines, andresidual tar exiting from separating device 50 is introduced into areaction zone 90 whereby the residual tar is removed. In one embodimentas depicted in FIG. 1 where the reaction zone 90 comprising one or morecatalytic bed 100, the gas stream comprising synthesis gas and residualtar exiting from separating device 50 are heated up to 1800° F. throughheat cross-exchanger 150 prior to entering the catalytic bed 100. Thecatalytic bed 100 may be a catalytic fluidized or bubble bed comprisingone or more tar-destruction catalysts whereby the residual tar isdecomposed. Heat required to bring the syngas up to reaction temperaturecan be supplied by external cross exchange or by injecting an oxygen andsteam mixture into the syngas stream. The carbon fines are eitherconverted to carbon monoxide or form particulates to pass upward throughsaid catalytic fluidized bed 100 along with outlet gaseous productscomprising synthesis gas. The catalytic bed 100 may also be catalyticfixed bed comprising one or more tar-destruction catalysts whereby theresidual tar is decomposed. In such case, the carbon fines are removedby a particulate filter prior to the catalyst bed. With either catalyticfluidized bed or catalytic fixed bed, the hot synthesis gas exiting thereaction zone 90 is cooled by heat cross-exchanger 150 with the colderinlet gas stream (or gas stream exiting from device 50) to recover theheat. According to one embodiment, the cooled gas stream exiting heatcross-exchanger 150, which is tar-free at this point, is then passed toa particulate filter 110 whereby the particulates is removed. Thetar-destruction catalysts may be zeolite, supported nickel, limestone,or any mixtures thereof.

In another embodiment as depicted in FIG. 2 which the reaction zone 90comprising one or more sorbent bed 120, the gas stream comprisingsynthesis gas and residual tar exiting from separating device 50 areintroduced into the sorbent bed 120 comprising one or more tar-absorbentsorbent whereby the residual tar is absorbed before the residualparticulates are filtered by in-situ filter 140, and recycled back tothe 1^(st) stage reactor. The sorbent bed 120 may be a fluidizedactivated carbon bed comprising one or more tar-absorbent sorbent. Thesorbent bed 120 may also be a fixed activated carbon bed comprising oneor more tar-absorbent sorbent. Since the absorption capacity ofactivated carbon improves at lower temperatures, internal cooling device130 (e.g. steam-cooled, internal cooling coil, panel, or baffles) isprovided for the activated carbon bed to reduced the temperature of thegas stream exiting from separating device 50, and to maintain the bedtemperature at 400° F. to 500° F. A small slipstream of tar-ladenactivated carbon is removed from the bed either continuously orperiodically to be regenerated through regenerator 160, typically byheating the carbon to a higher temperature to desorb the tar. In thecase of a fixed bed absorber, two vessels in parallel could be arrangedfor one to be on line removing the tars and the other off line forcleaning. The gas stream exiting the reaction zone 90 at this point istar and particulates-free synthesis gas. The tar absorbent sorbent maybe activated carbon, zeolite, certain natural occurring silicates or anymixtures thereof.

The materials of construction of the gasification reactor 10 are notcritical. Preferably, but not necessarily, the reactor walls are steeland are lined with an insulating castable or ceramic fiber or refractorybrick, such as a high chrome-containing brick in the reactor lowersection 30 and a dense medium, such as used in blast furnaces andnon-slagging applications in the reactor upper section 40, in order toreduce heat loss and to protect the vessel from high temperature as wellas to provide for better temperature control, all of which arecommercially available from several sources. Use of this type of systemprovides the high recovery of heat values from the carbonaceous solidsused in the process. Optionally and alternatively, the walls may beunlined by providing a “cold wall” system for fired reactor lowersection 30 and, optionally, unfired upper section 40. The term “coldwall”, as used herein, means that the walls are cooled by an externalcooling jacket, as is known conventionally in the art for prior art coalgasification systems. In such a system, the slag freezes on the interiorwall and provides for protection of the metal walls of the coolingjacket.

The physical conditions of the reaction in the first stage of theprocess in the reactor lower section 30 are controlled and maintained toassure rapid gasification of the char at temperatures exceeding themelting point of ash produced by char gasification to produce a moltenslag from the melted ash having a slag viscosity not greater thanapproximately 250 poises. The physical conditions of the reaction in thesecond stage of the gasification process in the reactor upper section 40are controlled to assure rapid gasification and heating of the coalabove its range of plasticity. The temperature of fired reactor lowersection 30 is maintained from 1500° F. to 3500° F., preferably from2000° F. to 3200° F. and most preferably from 2400° F. to 3000° F. Atsuch temperatures in the first stage in the reactor lower section 30,ash formed by the gasification of char therein melts to form molten slagwhich falls through the tap hole and is further conditioned in unitsoutside the scope of this document. The gas mixture from the 1^(st)stage leaves in the rotating upwardly moving vortex of gases and charascending through the reactor lower section. The temperature of unfiredreactor upper section reactor 40 is maintained from 450° F. to 1500° F.,preferably from 500° F. to 1400° F. and most preferably from 550° F. to1300° F. The hot intermediate product flowing upward from fired reactorlower section 30 provides heat for the endothermic reactions occurringin the unfired upper reactor section 40.

The temperature of the effluent from the unfired reactor upper section40 and gas stream exiting separating device 50 are typically from about800° F. to about 1300° F. The gas stream exiting separating device 50 isheated up through heat cross-exchanger 150 before entering a reactionzone 90 for tar removal. In one embodiment, the temperature of reactionzone 90 comprising one or more catalytic bed is maintained from 700° F.to 1900° F., preferably from 1000° F. to 1700° F. and most preferablyfrom 1200° F. to 1600° F. In another embodiment, the temperature ofreaction zone 90 comprising one or more sorbent bed is maintained from200° F. to 1000° F. preferably from 250° F. to 600° F. and mostpreferably from 300° F. to 500° F.

The process of this invention is carried out at atmospheric or higherpressures. Generally, the pressure in reactor lower section 30 andreactor upper section 40 is from about 14.7 psig to about 2000 psig,preferably from 50 psig to 1500 psig and, most preferably, from 150 psigto 1200 psig. The pressure in reaction zone 90 comprising one or morecatalytic bed is from about 14.7 psig to about 1500 psig, preferablyfrom 50 psig to 1500 psig and most preferably from 150 psig to 1200psig. In another embodiment, the pressure in reaction zone 90 comprisingone or more sorbent bed is from about 14.7 psig to about 1500 psig,preferably from 50 psig to 1500 psig and most preferably from 150 psigto 1200 psig.

In the various embodiments of the present invention, the velocity or thefeed rate of gases and solids passing through the dispersion devices 60and/or 60 a, of the reactor lower section reactor 30 is kept between 20and 120 feet per second, and preferably between 20 and 90 feet persecond, and most preferably between 30 and 60 feet per second. Theresidence time of char in the reactor lower section 30 is kept between 2second and 10 seconds and preferably between 4 and 6 seconds. Thevelocity or the feed rate of the slurry stream passing through thefeeding device 80 and/or 80 a of the reactor upper section reactor 40 iskept between 5 feet per second, and 100 feet per second, preferablybetween 10 feet per second and 80 feet per second, and most preferablybetween 20 and 60 feet per second. The residence time in the reactorupper section 40 is maintained between 5 and 40 seconds.

The process is applicable to any particulate carbonaceous material.Preferably, however, the particulate carbonaceous material is coalwhich, without limitation, includes lignite, bituminous coal,sub-bituminous coal, or any combination thereof. Additional carbonaceousmaterials are coke from coal, coal char, coal liquefaction residues,particulate carbon, petroleum coke, carbonaceous solids derived from oilshale, tar sands, pitch, biomass, concentrated sewer sludge, bits ofgarbage, rubber and mixtures thereof. The foregoing exemplifiedmaterials can be in the form of comminuted solids, and for bestmaterials handling and reaction characteristics, as pumpable slurries ina liquid carrier.

The liquid carrier for carbonaceous solid materials can be any liquidwhich is capable of vaporizing and participating in the reactions toform desired gaseous products, particularly carbon monoxide andhydrogen. The most readily considered liquid carrier is water whichforms steam in lower reactor section 30. The steam is capable ofreacting with carbon to form gaseous products which are constituents ofsynthesis gas. In addition, liquids other than water may be used toslurry the carbonaceous material. Preferably, the liquid is water, butit may also be a hydrocarbon such as, for example, fuel oil, residualoil, petroleum, and liquid CO₂. When the liquid carrier is ahydrocarbon, additional water or steam may be added to providesufficient water for efficient reaction and for moderating the reactortemperature.

Any gas containing at least 20 percent oxygen may be used as theoxygen-containing gas fed to fired reactor lower section 30. Preferredoxygen-containing gases include oxygen, air, and oxygen-enriched air.

The concentration of particulate carbonaceous material in the carrierliquid as a slurry is only that necessary to have a pumpable mixture. Ingeneral, the concentration ranges up to 70 percent by weight of thesolid material. Preferably, the concentration of particulatecarbonaceous material in the slurry ranges from 30 percent to 70 percentby weight in both the first and second stages of the process. Morepreferably, the concentration of coal in aqueous slurry is between 45and 69 percent by weight.

When coal is the feedstock, it can be pulverized before being blendedwith a liquid carrier to form slurry, or ground together with the liquidmedia. In general, any reasonably finely-divided carbonaceous materialmay be used, and any of the known methods of reducing the particle sizeof particulate solids may be employed. Examples of such methods includethe use of ball, rod and hammer mills. While particle size is notcritical, finely divided carbon particles are preferred. Powdered coalused as fuel in coal-fed power plants is typical. Such coal has aparticle size distribution in which 90 percent by weight of the coalpasses through a 200 mesh sieve. A coarser size of 100 mesh averageparticle size can also be used for more reactive materials, providedstable and non-settling slurry can be prepared.

As used herein, the term “char” refers to unburned carbon and ashparticles that remain entrained within a gasification system afterproduction of the various products.

As used herein, the term “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itself,or any combination of two or more of the listed items can be employed.For example, if a composition is described as containing components A,B, and/or C, the composition can contain A alone; B alone; C alone; Aand B in combination; A and C in combination; B and C in combination; orA, B, and C in combination.

The scope of protection is not limited by the description set out above,but is only limited by the claims which follow, that scope including allequivalents of the subject matter of the claims. Each and every claim isincorporated into the specification as an embodiment of the presentinvention. Thus the claims are a further description and are an additionto the preferred embodiments of the present invention.

The discussion of a reference in the description of related art is notan admission that it is prior art to the present invention, especiallyany reference that may have a publication date after the priority dateof this application. The disclosures of all patents, patent applicationsand publications cited herein are hereby incorporated herein byreference, to the extent that they provide exemplary, procedural orother details supplementary to those set forth herein.

Any element in a claim that does not explicitly state “means for”performing a specified function, or “step for” performing a specificfunction, is not to be interpreted as a “means” or “step” clause asspecified 35 U.S.C. §112 ¶6. In particular, the use of “step of” in theclaims herein is not intended to invoke the provisions of 35 U.S.C. §112¶6.

What is claimed is:
 1. A process for gasification of a carbonaceousmaterial, comprising the steps of: a. introducing recycled char into areactor lower section and partially combusting said recycled chartherein with a stream comprising an oxygen-containing gas or steamthereby evolving heat and forming mixture products comprising synthesisgas and molten slag, wherein said reactor lower section is operated in arange of 2400° F. to 3000° F. and in a range of 150 psig to 1200 psig,wherein recycled char and said stream are introduced to said reactorlower section through one or more dispersion devices on said reactorlower section at a feed rate between 30 to 60 feet per second, and aresidence time of said recycled char in said reactor lower section of 4to 6 seconds; b. passing said synthesis gas from said reactor lowersection upward into a reactor upper section and pyrolysing therein witha stream comprising a slurry of particulate carbonaceous material in aliquid carrier, thereby forming mixture products comprising i) a gaseousproduct stream comprising synthesis gas and residual tar, ii) char, andiii) vapor, wherein said reactor upper section is operated in a range of550° F. to 1300° F., and in a range of 150 psig to 1200 psig, whereinsaid slurry of particulate carbonaceous material in said liquid carrieris introduced to said reactor upper section through one or more feeddevices on said reactor upper section at a feed rate between 20 to 60feet per second, and wherein said slurry has a residence time in saidreactor upper section of 5 to 40 seconds; c. passing said mixtureproducts from said reactor upper section through a separation device,wherein said char is separated from said mixture products from saidreactor upper section and introduced to said reactor lower section assaid recycled char of step a), wherein said recycled char is dry and isthe only feedstock introduced to said reactor lower section; and d.introducing said gaseous product stream from said reactor upper sectioncomprising synthesis gas and residual tar into one or more catalyticbeds comprising one or more catalysts selected from the group consistingof zeolite, supported nickel, limestone, and mixtures thereof, whereinsaid catalytic bed is operated between 1200° F. and 1600° F. and between150 psig and 1200 psig, wherein said residual tar is decomposed, whereinsaid heat evolved in said step (a) is recovered by converting saidslurry of particulate carbonaceous material and said carrier liquid instep (b) into said gaseous product stream in step (b).